Nanowhiskers with PN junctions, doped nanowhiskers, and methods for preparing them

ABSTRACT

Nano-engineered structures are disclosed, incorporating nanowhiskers of high mobility conductivity and incorporating pn junctions. In one embodiment, a nanowhisker of a first semiconducting material has a first band gap, and an enclosure comprising at least one second material with a second band gap encloses said nanoelement along at least part of its length, the second material being doped to provide opposite conductivity type charge carriers in respective first and second regions along the length of the of the nanowhisker, whereby to create in the nanowhisker by transfer of charge carriers into the nanowhisker, corresponding first and second regions of opposite conductivity type charge carriers with a region depleted of free carriers therebetween. The doping of the enclosure material may be degenerate so as to create within the nanowhisker adjacent segments having very heavy modulation doping of opposite conductivity type analogous to the heavily doped regions of an Esaki diode. In another embodiment, a nanowhisker is surrounded by polymer material containing dopant material. A step of rapid thermal annealing causes the dopant material to diffuse into the nanowhisker. In a further embodiment, a nanowhisker has a heterojunction between two different intrinsic materials, and Fermi level pinning creates a pn junction at the interface without doping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/230,086, filed Aug. 22, 2008, which is a Divisional of U.S.application Ser. No. 10/814,630, filed Apr. 1, 2004, now U.S. Pat. No.7,432,522, which claims the benefit priority of U.S. Provisional PatentApplication No. 60/459,990, filed Apr. 4, 2003, the entirety of all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to structures and devices produced bytechniques of nanotechnology.

More specifically, the invention relates to such structures and devicesincorporating at least one element, essentially in one-dimensional form,and that is of nanometer dimensions in its width or diameter, and thatpreferably is produced by the so-called Vapor-Liquid-Solid (VLS)mechanism. For the purposes of this specification, such element will betermed a “nanowhisker”.

2. Brief Description of the Prior Art

Nanotechnology covers various fields, including that of nanoengineering,which may be regarded as the practice of engineering on the nanoscale.This may result in structures ranging in size from small devices ofatomic dimensions, to much larger scale structures for example on themicroscopic scale. Commonly, such structures include nanostructures. Incertain contexts nanostructures are considered to be those having atleast two dimensions not greater than about 100 nm, with some authorsusing the term to identify structures having at least two dimensions notgreater than about 200 nm. Nevertheless, some procedures for fabricatingsuch small structures are useful for structures having at least twodimensions somewhat greater, e.g., structures having at least twodimensions not greater than about 1 micrometer (μm). Ordinarily, layeredstructures or stock materials having one or more layers with a thicknessless than 1 μm are not considered to be nanostructures. Thus, althoughthe term “nanostructure” is more classically considered to refer tostructures having at least two dimensions not greater than about 100 nm,in the following discussion, the term “nanostructure”, “nanowhisker”, or“nanoelement” is intended to include a structure having at least twodimensions not greater than about 1 μm.

Nanostructures include so-called one-dimensional nanoelements,essentially in one-dimensional form, that are of nanometer dimensions intheir width or diameter, and that are commonly known as nanowhiskers,nanorods, nanowires, nanotubes, etc.

As regards nanowhiskers, the basic process of whisker formation onsubstrates, by the so-called VLS (vapor-liquid-solid) mechanism, is wellknown. A particle of a catalytic material, usually gold, for example, ona substrate is heated in the presence of certain gases to form a melt. Apillar forms under the melt, and the melt rises up on top of the pillar.The result is a whisker of a desired material with the solidifiedparticle melt positioned on top. (See E. I Givargizov, Current Topics inMaterials Science, Vol. 1, pages 79-145, North Holland PublishingCompany, 1978.) The dimensions of such whiskers were in the micrometerrange.

Although the growth of nanowhiskers catalyzed by the presence of acatalytic particle at the tip of the growing whisker has conventionallybeen referred to as the VLS (Vapor-Liquid-Solid) process, it has come tobe recognized that the catalytic particle may not have to be in theliquid state to function as an effective catalyst for whisker growth. Atleast some evidence suggests that material for forming the whisker canreach the particle-whisker interface and contribute to the growingwhisker even if the catalytic particle is at a temperature below itsmelting point and presumably in the solid state. Under such conditions,the growth material, e.g., atoms that are added to the tip of thewhisker as it grows, may be able to diffuse through a the body of asolid catalytic particle or may even diffuse along the surface of thesolid catalytic particle to the growing tip of the whisker at thegrowing temperature. Evidently, the overall effect is the same, i.e.,elongation of the whisker catalyzed by the catalytic particle, whateverthe exact mechanism may be under particular circumstances oftemperature, catalytic particle composition, intended composition of thewhisker, or other conditions relevant to whisker growth. For purposes ofthis application, the term “VLS process”, “VLS method”, or “VLSmechanism”, or equivalent terminology, is intended to include all suchcatalyzed procedures wherein nanowhisker growth is catalyzed by aparticle, liquid or solid, in contact with the growing tip of thenanowhisker.

International Application Publication No. WO 01/84238 discloses in FIGS.15 and 16 a method of forming nanowhiskers wherein nanometer sizedparticles from an aerosol are deposited on a substrate and theseparticles are used as seeds to create filaments or nanowhiskers.

For the purposes of this specification the term nanowhiskers is intendedto mean “one-dimensional” nanoelements with a width or diameter (or,generally, a cross-dimension) of nanometer size, the elements havingbeen formed by the so-called VLS mechanism. Nanowhiskers are alsoreferred to in the art as “nanowires” or, in context, simply as “wires”,and such terminology, as used in this application, is equivalent to theterm “nanowhiskers”.

Several experimental studies on the growth of nanowhiskers have beenmade, including those reported by Hiruma et al. They grew III-Vnano-whiskers on III-V substrates in a metal organic chemical vapordeposition (MOCVD) growth system. (See K. Hiruma, et al., J. Appl. Phys.74, page 3162 (1993); K. Hiruma, et al., J. Appl. Phys. 77, page 447(1995); K. Hiruma, et al., IEICE Trans. Electron. E77C, page 1420(1994); K. Hiruma, et al., J. Crystal Growth 163, pages 226-231 (1996)).

Hiruma et al. have made pn junctions within nanowhiskers by doping GaAswhiskers with Si using Si₂H₆, during the growth process, and switchingthe dopant to an opposite conductivity type (carbon) during growth: K.Hiruma et al., J. Appl. Phys. 77(2), 15 Jan. 1995 p. 447, see pages459-461; and K. Hiruma et al J. Appl. Phys. 75(8) 4220 (1994). Ingeneral, there are problems in that the definition of the junctionwithin the nanowire is not good enough for electrical components, and inthat the presence of dopant ions within the crystal creates crystalimperfections and reduces carrier mobility.

In another approach by Lieber et al, WO-A-03/005450, nanowires wereproduced, different wires being doped with opposite conductivity typedopants, and two wires of opposite conductivity type were physicallycrossed, one on top of the other, so that a pn junction was formed attheir point of contact. A difficulty with this approach is the extrastep required of physically positioning the nanowires.

In planar semiconductor processing, various doping techniques are known.One technique that is valuable with heterojunctions is known asmodulation doping. In this technique, carriers from a doped layer of,e.g., AlGaAs, diffuse across an interface with an undoped material,e.g., GaAs, and form a very thin layer of carriers of very highmobility, within a potential well, next to the interface—see for exampleFIG. 1 of WO 02/19436.

U.S. Pat. No. 5,362,972 discloses an FET wherein the current flowpathbetween source and drain is composed of GaAs nanowhiskers. Thenanowhiskers are surrounded by n-doped AlGaAs, to create by modulationdoping a one-dimensional electronic gas within each nanowhisker.

WO 02/020820 discloses a modulation doping technique in CoaxialHeterostructure Nanowires, wherein dopants in an outer coaxial layerdonate free carriers to an inner nanowire.

In other techniques, doping of a semiconductor region in a planarsemiconductor device occurs by diffusion of ions from an adjacent regionof polymer; see Guk et al., Semiconductors Vol. 33(3), pp. 265-275,March 1999.

In co-pending U.S. patent application Ser. No. 10/613,071, filed on Jul.7, 2003, in the names of Samuelson and Ohlsson, the contents of whichare incorporated herein by reference, a process was disclosed forproducing nanowhiskers, and structures were disclosed incorporatingnanowhiskers.

SUMMARY OF THE INVENTION

It is an object of the invention to provide new and improvednano-engineered structures incorporating nanowhiskers and otherone-dimensional nanoelements, the nanoelements having improvedconductivity characteristics.

It is a further object of the invention to provide new and improvednano-engineered structures incorporating nanowhiskers and otherone-dimensional nanoelements, wherein the nanoelements contain improvedpn junctions.

In at least a preferred embodiment of the invention, nanowhiskers orother one-dimensional nanoelements are grown as pure crystals withoutdoping. The nanowhisker is then enclosed in an enclosure comprising asurrounding layer or matrix of a further different material that willusually be a semiconductor material. Dopant ions are incorporated intothis further material, by an appropriate process during or after itsdeposition. Carriers liberated in the further material transfer into thenanowhisker. The band structures of the nanoelement and the furthermaterial ensure that it is energetically favorable for the carriers todiffuse into the nanoelement; this is effectively by the process knownas modulation doping wherein a potential well is defined within thenanowhisker. Thus effectively the nanowhisker is doped with carriers,but that these are of high mobility, since the absence of dopant ionswithin the nanowhisker ensures that the crystalline structure is notdeformed.

The preferred embodiment provides a method of producing aone-dimensional nanoelement of desired conductivity, the methodcomprising the steps of (1) forming a one-dimensional nanoelement of afirst material, (2) surrounding the nanoelement with a second material,different from that of the nanoelement, the second material containingdopant material whereby charge carriers from the dopant material diffuseinto the nanowhisker to create said desired conductivity. Morespecifically, the method comprises the steps of (1) forming by the VLSmethod a nanowhisker on a substrate, the nanowhisker including a firstsemiconducting material, and (2) forming a coaxial layer around thenanowhisker of a second semiconducting material, and (3) incorporatingdopant material into the coaxial layer whereby charge carriers from thedopant material diffuse into the nanowhisker to create said desiredconductivity.

The invention provides a means of creating a pn junction within aone-dimensional nanoelement by modulation doping.

Specifically the invention provides a nanoengineered structure includinga one-dimensional nanoelement of a first semiconducting material havinga first bandgap, an enclosure comprising at least one second materialhaving a second bandgap enclosing said nanoelement along at least partof its length, and said second material being doped to provide oppositeconductivity type charge carriers in respective first and second regionsalong the length of the of the nanowhisker, whereby to create bytransfer of charge carriers into said nanoelement, corresponding firstand second regions of opposite conductivity type charge carriers with apn junction therebetween in said nanoelement, and wherein the bandgapsare such that it is energetically favorable for the charge carriers toremain in said nanoelement.

The enclosure for the nanoelement may be a coaxial jacket. In onepreferred form a thin nanowhisker of GaAs is grown, then the growthconditions are changed from those appropriate for catalytic growth tothose appropriate for bulk growth, so that a coaxial jacket is formedaround the sides of the nanowhisker. The material may be AlGaAs. It isnecessary to dope the lower part of the AlGaAs jacket with oneconductivity type dopant material, and the upper parts of the coaxialjacket with opposite conductivity dopant ions. One exemplary techniquefor achieving this is to embed the coaxial jacket within a polymermatrix comprising upper and lower layers as, for example, spin on glassor polymer substances. The lower layer has one conductivity type dopantmaterial, and the upper layer has the opposite conductivity type dopantmaterial. Rapid thermal annealing causes diffusion of the dopantmaterial into the coaxial jacket. The thermal annealing step is stoppedbefore appreciable diffusion into the one-dimensional nanoelement. Thepresence of the dopant ions within the coaxial jacket creates modulationdoping within the nanowhisker, and a pn junction between the two regionsof opposite conductivity type material. The space charge within eachregion is maintained within the nanoelement, and the depletion region ofthe pn junction may be sharp or as diffuse as desired (typically withinthe range 50 nm to 1 μm). The diameter of the nanowhisker is preferablysmall, about 20 nm. The coaxial jacket may be as small as 10 nm thick,but it may in other cases be preferable to have a jacket that is 200 nmthick or one that even fills the volume between the nanowires completelyin an array of nanowires.

The materials of the nanowire and jacket may be GaAs and AlGaAs, forexample. Other material combinations could be InAs in the core sectionand AlSb in the surrounding material or a germanium core and a siliconjacket.

In a modification, the coaxial jacket is doped in one conductivity typeduring its formation. A layer of spin-on glass is then formed partwayalong the length of the nanowire, containing opposite type conductivityions, that are sufficiently concentrated to reverse the conductivitytype in the lower part of the coaxial jacket. In a further modification,the coaxial jacket is grown in an undoped condition, and a layer ofspin-on glass is then formed partway along the length of the nanowire,containing one type conductivity ions. The structure is then exposed toa gas containing opposite type conductivity ions that diffuse into theupper part of the coaxial jacket, where they create a region of oppositeconductivity type. The ions in the gas also diffuse into the layer ofspin on glass, but not in a sufficient concentration to overcome theexisting concentration of the one type conductivity. A pn junction isthereby formed in the nanowire.

In an alternative form, the one-dimensional nanoelement is encapsulatedwithin an enclosure formed by first and second layers of polymermaterial or spin on glass, each layer having opposite conductivitydopant material. Direct charge transfer of the carriers from the polymermatrix creates modulation doping within the nanowhisker, and twoseparate regions of oppositely signed charge carriers with a pn junctionbetween them.

In a further alternative form of the invention the doping is so heavy asto create degenerate doping within the nanoelement, that is to say theFermi level exists, in one region, in the conduction band, and in theother region, in the valence band. In this state, the nanoelementcomprises a tunnel diode or Esaki diode wherein in known manner, forwardbiasing of the junction creates a negative resistance caused bytunneling between the valence and the conduction bands.

In a further aspect, a nanowhisker or other one-dimensional nanoelementis surrounded by a material containing dopant ions. For example thesurrounding material may be a polymer material. By a process, forexample a subsequent step of rapid thermal annealing, the dopant ions inthe matrix material are permitted themselves to diffuse into thenanowhisker, to create a desired conductivity. This provides advantagesover a direct doping into the nanoelement, by providing an extra degreeof control over the doping process, and permitting the diffusion ofcertain dopants into the nanoelement that would not be possible by amore direct process. Although a polymer material is preferred, which isevaporated or spun onto a substrate so as to surround the nanowhisker,other materials may be employed, such as for example semiconductormaterial or dielectric material grown onto the substrate. The dopantmaterial may be incorporated in the surrounding material beforeapplication to the substrate, during the application to the substrate,or as a subsequent step after the surrounding material is formed on thesubstrate.

Specifically, the invention provides a method of forming aone-dimensional nanoelement of a desired conductivity, comprising:

-   -   (a) forming a one-dimensional nanoelement on a substrate, the        nanoelement being formed of a first material;    -   (b) forming at least a first layer of a further material on the        substrate and surrounding, at least partially, the nanoelement,        the further material having a first conductivity type dopant        material therein, and    -   (c) processing the further material so that said dopant material        diffuses into the nanoelement, whereby to create a desired        conductivity therein.

Additionally, the invention provides a nanoengineered structure,comprising a one-dimensional nanoelement, of a first material, disposedon a substrate, and at least a first layer of material formed on thesubstrate and surrounding, at least partially, the nanoelement, thefirst layer having a first conductivity type dopant material therein,said first conductivity type dopant material having diffused into thenanoelement, whereby to create a desired conductivity within thenanoelement.

In a further aspect, a pn junction is created within a one-dimensionalnanoelement, preferably a nanowhisker. A nanowhisker is grown on asubstrate, and embedded in a surrounding material. The material consistsof first and second layers, formed on the substrate, one on top of theother, for example as polymer layers evaporated or spun onto thesubstrate. Alternatively the layers may be of some other material, forexample dielectric material or semiconducting material grown on thesubstrate. The first layer extends partway up the nanowhisker, and has afirst dopant material incorporated within it or subsequently injected,providing charge carriers of a first type. The second layer extendstowards the top of the nanowhisker, and has a second dopant materialcontained within it or subsequently injected into it, providing chargecarriers of an opposite conductivity type. The surrounding material istreated, as for example by rapid thermal annealing, so that the dopantions themselves diffuse into the respective first and second regions ofthe nanoelement, to create an effective pn junction within thenanowhisker. In this case, the surrounding layers may be commerciallyavailable polymer layers evaporated or spun onto the substrate. Thedopant materials are incorporated into the polymer materials before,during, or after the application of the polymer materials to thesubstrate.

In either case, the effective pn junction can be made as sharp asdesired, approaching that of a few nanometers. More than one pn junctionmay be created by employing multiple layers, each layer havingappropriate dopant material.

Specifically the invention provides a nanoengineered structure includinga one-dimensional nanoelement with at least one pn junction therein,comprising a nanowhisker upstanding from a substrate, and a first layerof a material formed on the substrate and surrounding and extendingpartway up the nanowhisker, the first layer having a first conductivitytype dopant material therein, and a second layer of material formed ontop of the first layer and surrounding and extending towards the top ofthe nanowhisker, and having a second conductivity type dopant materialtherein, whereby to create by diffusion from said first and secondlayers into respective first and second regions of the nanowhisker, a pnjunction within the nanowhisker between the first and second regions.

The invention also provides a method of forming a one-dimensionalnanoelement with a pn junction therein, comprising:

(a) forming a nanowhisker upstanding from a substrate,

(b) forming a first layer of material on the substrate and surroundingand extending partway up the nanowhisker, the first layer having a firstconductivity type dopant material therein, and

(c) forming a second layer of material on top of the first layer andsurrounding and extending towards the top of the nanowhisker, and havinga second conductivity type dopant material therein, so that diffusionfrom the first and second layers into respective first and secondregions of the nanowhisker creates a pn junction within the nanowhiskerbetween the first and second regions.

In a fifth aspect, the invention recognises that there are problems inchemical doping of dopant ions in a nanowhisker of III-V semiconductormaterial, since for most III-V semiconductors, solid solubility at roomtemperature is limited, and during cooling out-diffusion is fast withthese nanodimensions. Thus the amount of doping within the nanowhiskermay be difficult to accurately predetermine. The invention recognisesthat the interface between a nanoelement and a surrounding medium, or aheterojunction interface within a nanoelement, may have a stronger andmore significant role than hitherto realised in determining theelectrical characteristics of the nanoelement.

It is known that localised surface “trap” states exist at the surfacesof bulk semiconductors; this is exhibited for example in Schottkydiodes. This creates what is known as Fermi Level Pinning, where thesurface trap states determine the relative levels of the conduction andvalence bands in the junction materials. See, e.g., “DefectiveHeterojunction Models”, Freeouf J L, Woodall J M,: IBM Corp,: SurfaceScience, 1986, V 168, N1-3, P 518-530.

The invention recognises that for a one-dimensional nanoelement, wherethere may be a wide range of possibilities to combine III-Vsemiconductors in spite of lattice mismatch, Fermi Level Pinning is aconstructive way to make pn-junctions by choosing semiconductor alloycomposition to determine carrier type. In such devices the band gap canbe engineered and the carrier type can be controlled to make new typesof semiconductor devices. When a semiconductor crystal ends abruptly atan interface, and “band bending” tends to occur to equalize the FermiLevels on the two sides of the interface, Fermi Level Pinning, arisingfrom the existence of surface trap states, counteracts this effect toreduce the amount of charge transfer across the interface.

The invention further provides a one-dimensional nanoelement including afirst segment of a first semiconductor crystalline material, and asecond segment of a second semiconductor crystalline material differentfrom that of the first, and with a heterojunction therebetween, wherebythe first and second materials are selected such that charge carriers ofopposite conductivity type are provided at the opposite sides of theheterojunction interface so as to create a pn junction withpredetermined characteristics, which characteristics are at leastpartially determined by Fermi level pinning.

The invention also provides a method of forming a pn junctioncomprising:

a. forming a one dimensional nanoelement having a first segment of afirst crystalline material, and a second segment of a second crystallinematerial different from that of the first, with a heterojunctiontherebetween,

b. the first and second materials being selected so as to provide chargecarriers of opposite conductivity type at the heterojunction so as tocreate a pn junction with predetermined characteristics, whichcharacteristics are at least partially determined by Fermi levelpinning.

In accordance with the invention, the charge carriers can be provided bythe intrinsic nature of the first and second materials. For III-Vmaterials, stoichiometric compositions of ternary or quaternarymaterials can be chosen for desired conductivity characteristics.

It has been found that the present invention is particularly applicableto III-V compounds epitaxially grown (CBE or MOCVD, or MOVPE), undergroup III rich conditions. Under these conditions, the outermost atomicsurface layers may have excess Ga or In ions and these create defectstates, as further described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic of the CBE apparatus used for making the describedembodiments.

FIG. 2 is a cross-sectional schematic view showing a step in theformation of the first embodiment of the invention, with an accompanyingenergy band diagram.

FIG. 3A shows a cross-section of a nanowhisker prepared by the VLSmethod.

FIG. 3B shows a cross-section of a nanowhisker having an enclosure orjacket according to the invention.

FIG. 3C shows an array of such clad nanowhiskers extending from a (111)surface.

FIG. 3D shows an enlarged view of a nanowhisker having been separatedfrom the surface.

FIG. 3E is a view of the cross-section of the clad nanowhisker showing ahexagonal structure that is characteristic of nanowhiskers growing in a<111> direction.

FIG. 3F is a luminescence curve showing characteristic peaks atapproximately 1.5 and 1.8 eV, which represent GaAs and AlGaAs materialsrespectively.

FIG. 4 is a cross-sectional view of a second embodiment of theinvention.

FIG. 5 is a cross-sectional schematic view of a third embodiment of theinvention.

FIG. 6 is a cross-sectional schematic view of a fourth embodiment of theinvention.

FIG. 7 is a cross-sectional schematic view of a fifth embodiment of theinvention.

FIG. 8 is a cross-sectional schematic view of a sixth embodiment of theinvention.

FIG. 9 is a graph showing the energy levels of the bands and surfacestates of a number of semiconductor materials.

FIG. 10 is a schematic energy level diagram for the doped nanowhiskersof FIG. 7 and FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments to be described are all formed with nanowhiskers,preferably according to the Chemical Beam Epitaxy method (CBE) describedin copending U.S. patent application Ser. No. 10/613,071 filed Jul. 7,2003, the contents of which are herein incorporated by reference.

As indicated above, in the following detailed description of theinvention, the term “nanoengineered structures” signifies a structurethat includes structures, e.g., elements, parts, or the like, havingdimensions as defined above, i.e., structures having at least twodimensions less than about 1 micrometer. Such structures are referred toherein as “nanoelements” or nanostructures, and/or, because of theirgenerally elongated shape, as “nanowhiskers” or “nanowires”.

Chemical Beam Epitaxy (CBE) combines a beam epitaxial technique likeMolecular Beam Epitaxy (MBE) and the use of chemical sources similar toMetal Organic Chemical Vapor Deposition (MOCVD). In MOCVD or relatedlaser ablation techniques, the pressure inside the reactor is usuallygreater than 10 mbar and the gaseous reactants are viscous, which meansthat they have a relatively high resistance to flow. The chemicals reachthe substrate surface by diffusion. CBE reduces the pressure to lessthan 10⁻⁴ mbar and the mean free path of the diffusants then becomeslonger than the distance between the source inlet and the substrate. Thetransport becomes collision free and occurs in the form of a molecularbeam. The exclusion of the gas diffusion in the CBE system means a fastresponse in the flow at the substrate surface and this makes it possibleto grow atomically abrupt interfaces.

The CBE apparatus shown in FIG. 1 consists of a UHV growth chamber 100where the sample 102 is mounted on a metal sample holder 104 connectedto a heater 106. Around the chamber there is a ring 108 filled withliquid nitrogen that is called the cryoshroud. The cryoshroud pumps awayspecies that don't impinge or that desorb from the substrate surface. Itprevents contamination of the growing surface layer and reduces thememory effect. Vacuum pumps 110 are provided.

The sources 112 for CBE are in liquid phase and they are contained inbottles which have an overpressure compared to the chamber. The sourcesare usually as follows: TMGa, TEGa, TMIn, TBAs, and TBP. The bottles arestored in constant-temperature baths and by controlling the temperatureof the liquid source, the partial pressure of the vapor above the liquidis regulated. The vapor is then fed into the chamber through a pipecomplex 114 to, in the end of the pipe just before the growth chamber, asource injector 116. The source injector is responsible for injection ofthe gas sources into the growth chamber 100, and for generation of amolecular beam with stable and uniform intensity. The III-material, fromthe metal organic compounds TMIn (trimethylindium), TMGa(trimethylgallium) or TEGa (triethylgallium), will be injected by lowtemperature injectors to avoid condensation of the growth species. Theywill decompose at the substrate surface. The V-material is provided bythe metal-organic compounds, TBAs (tertiarybutylarsine) or TBP(tertiarybutylphosphine). As opposed to the decomposition of theIII-material, the V-material will be decomposed before injection intothe growth chamber 100, at high temperatures, in the injectors 116.Those injectors 116 are called cracking cells and the temperatures arekept around 900° C. The source beam impinges directly on the heatedsubstrate surface. Either the molecule gets enough thermal energy fromthe surface substrate to dissociate in all its three alkyl radicals,leaving the elemental group III atom on the surface, or the molecule getdesorbed in an undissociated or partially dissociated shape. Which ofthese processes dominates depends on the temperature of the substrateand the arrival rate of the molecules to the surface. At highertemperatures, the growth rate will be limited by the supply and at lowertemperatures it will be limited by the alkyl desorption that will blocksites.

This Chemical Beam Epitaxy method permits formation of heterojunctionswithin a nanowhisker, which are abrupt, in the sense there is a rapidtransition from one material to another over a few atomic layers.

Referring now to FIG. 2, a first embodiment of the invention is formedby positioning a gold aerosol particle 2 on a III-V substrate 4, e.g., agallium arsenide substrate. With appropriate conditions of temperatureand pressure a nanowhisker of indium arsenide is grown by injectingorganic materials TMIn and TBAs in a conventional VLS procedure, e.g.,in a chemical beam epitaxial method, using the apparatus describedabove, or by a metal organic vapor phase epitaxy (MOVPE), or the like.Indium and arsenide ions are absorbed in the gold particle 2 andsupersaturation conditions create a solid pillar 6 of indium arsenide.

Once the indium arsenide whisker has been grown, different materialsTEGa and TBP are used to create a coaxial jacket or surrounding layer 8of GaP around the nanowhisker 6. Layer 8 may be created by CBE; usingthe apparatus of FIG. 1, wherein the conditions of temperature (106)and/or pressure (112) are changed to inhibit growth by the VLSmechanism, and instead to support bulk growth. Alternatively the goldmelt particle 2 can be removed mechanically, so that subsequent growthof GaP will occur in bulk form.

The resulting energy level bandgap diagram is shown with an energy gapof 2.3 EV separating the conduction bands for gallium phosphide, whereasthere is a bandgap of 0.3 EV for the central indium arsenide whisker.

The jacket or shell material (GaP in this case) may then be doped, e.g.,via the vapor phase, resulting in a sheath at the periphery of the GaPjacket which will contain donor dopants such as tellurium.

As an alternative to tellurium, any donor dopant materials that arecommonly used for GaP may be used, see for example CRC The Handbook ofChemistry and Physics, Semiconductor Properties, e.g., Si, Sn, Te, Se,S, or the like. Alternatively, if an acceptor-doped jacket or shell isdesired, appropriate acceptor materials, e.g., Zn, Fe, Mg, Be, Cd, orthe like, can be incorporated.

As an alternative to InAs/GaP, any other combination of materials may beused, subject to the bandgaps providing energetically favorableconditions—the band gap of the surrounding layer should be wider thanthat of the nanowhisker; thus for a whisker of InAs, covering materialsof GaAs, GaP or InP may be employed—see for example CRC The Handbook ofChemistry and Physics, Semiconductor Properties.

The effect of doping with tellurium ions is to liberate charge carrierelectrons within gallium phosphide layer 8. These electronspreferentially transfer into the central nanowhisker, where the energystates (conduction band/valence band levels) determine that theelectrons are in energetically favorable condition. The theory isessentially that of modulation doping that is a technique employed inplanar technology as described in WO 02/1438.

This therefore creates a nanowhisker with a desired electricalconductivity. The nanowhisker also has a high mobility because there areno dopant ions within the crystal lattice deforming the latticestructure.

Referring now to FIGS. 3A to 3F there is shown a specific example of thefirst embodiment of the invention. Nanowhiskers 6 of gallium arsenidewere grown from gold catalytic particles by an epitaxial process from aGaAs substrate having a (111) surface. The growth conditions were thenchanged by altering the temperature, and modifying the gaseous pressureof the As-containing gas, so as to grow epitaxially, by bulk growthrather than catalytic growth, material of AlGaAs along the side of theGaAs nanowhiskers. The result as shown in FIGS. 3B and 3C are cylinders,in the form of a candle, with an inner core 6 of a GaAs 20 nanometers indiameter, and an outer cladding 8 of AlGaAs between 100 and 5000nanometers in diameter.

FIG. 3C shows an array of such clad nanowhiskers extending from a (111)surface.

FIG. 3D shows an enlarged view of a nanowhisker having been separatedfrom the surface.

FIG. 3E is a view of the cross-section of the clad nanowhisker showing ahexagonal structure that is characteristic of nanowhiskers growing in a<111> direction.

FIG. 3F is a luminescence curve showing characteristic peaks atapproximately 1.5 and 1.8 eV, which represent GaAs and AlGaAs materialsrespectively. An intermediate hump is thought to be caused by spatiallyindirect transitions.

Referring now to FIG. 4 there is shown a second embodiment of theinvention. Similar parts to those of FIG. 2 are identified by the samereference numeral. A structure is produced comprising an innernanowhisker of GaAs material 6, produced from a catalytic particle 2.The whisker is surrounded by a coaxial jacket 8 of AlGaAs. Then, firstand second layers 20, 22 are provided, being of polymer or glassmaterial spun on to the surface of substrate 4. Layer 20 contains n typedopant ions 24, and layer 22 contains p type dopant ions 26. A rapidthermal annealing step ensures that dopant ions 24, 26 migrate intocorresponding regions 28, 30 of coaxial jackets 8. The annealing step iscontrolled such that there is no appreciable diffusion into thenanowhisker 6.

The result is that the dopant ions within regions 28, 30 createcorresponding regions 32, 34 within nanowhisker 6 by modulation dopingof opposite conductivity type. These regions that are stable spacecharge regions create a region 36 depleted of free carriers resemblingthe depletion region of a pn junction between semiconductor materials ofopposite conductivity type.

The level of dopant concentration within regions 28, 30 may be such thathighly degenerative doping is produced, with correspondingly heavymodulation doping of the segments 32, 34 of the nanowhisker. Such heavymodulation doping may create a condition analogous to that existing inan Esaki or tunnel diode, with corresponding tunneling between theregions and an associated negative resistance effect.

Referring now to FIG. 5, there is shown a third embodiment of theinvention, wherein similar parts to those of FIG. 2 are denoted by thesame reference numeral. Thus an indium arsenide nanowhisker 6 is grownon a gallium arsenide substrate 4 by chemical beam epitaxy employing agold catalytic particle 2.

After formation of the nanowhisker, a first layer 50 of polymer materialis evaporated (preferred) or spun onto the substrate 4. There iscommercially available a wide range of dielectric materials formed ofcarbon or silicon based polymers, some of which are doped and havedefined electrical conductivity characteristics. The polymer materialhas contained within it a desired concentration of dopant ions of adesired type. As may be seen, layer 50 extends towards the top of thenanowhisker. The depth of layer 50 can be determined very accuratelywith evaporation of polymer.

The entire structure is then subject to rapid thermal annealing. Thispermits the dopant ions in the polymer material layer 50 to diffuse intothe nanowhisker regions 54, to provide a controlled doping of theregions 54. The temperature of the annealing step depends on thematerials employed.

There is thus provided a nanowhisker with a desired degree ofconductivity, the method of doping providing a high degree of controlover the conductivity.

Referring now to FIG. 6, there is shown a fourth embodiment of theinvention, wherein similar parts to those of FIG. 2 are denoted by thesame reference numeral. Thus an indium arsenide nanowhisker 6 is grownon a gallium arsenide substrate 4 by chemical beam epitaxy employing agold catalytic particle 2.

After formation of the nanowhisker, a first layer 60 of polymer materialis evaporated (preferred) or spun onto the substrate 4. There iscommercially available a wide range of dielectric materials formed ofcarbon or silicon based polymers, some of which are doped and havedefined electrical conductivity characteristics. The polymer materialhas contained within it a desired concentration of dopant ions of adesired type. As may be seen, layer 60 extends roughly halfway along thelength of the nanowhisker. Thus, for a nanowhisker that is 2 micrometerslong, the depth of layer 60 is 1 micrometer. The depth can be determinedvery accurately with evaporation of the polymer.

A second layer 62 of polymer material of the same type as the first buthaving a dopant material of opposite conductivity type is evaporated onto layer 60 and extends up to the top of the nanowhisker, to a heightapproximately the same as the gold particle 2.

The entire structure is then subject to rapid thermal annealing. Thispermits the dopant ions in the polymer material layer 60 to diffuse intothe adjacent nanowhisker region 64, to provide a controlled doping ofthe region 64. Further, the dopant ions in the polymer material layer 62diffuse into the adjacent nanowhisker region 66, to provide a controlleddoping of the region 66. The temperature of the annealing step dependson the materials employed.

Thus, region 64 of whisker 6 may contain for example negative chargecarriers, whereas positive charge carriers from layer 62 are containedin region 66 of whisker 6. This effectively creates a pn junction 68between the two regions 64, 66.

The junction 68 may be sharply defined within the nanowhisker. For typesof dopant materials, any of the commonly used materials may be used.See, e.g., CRC The Handbook of Chemistry and Physics, SemiconductorProperties.

Three or more layers of polymer may be deposited, each with appropriatedopant materials. This permits the formation of multiple pn junctionswithin the whisker.

Referring now to FIGS. 7 to 10, there is shown fifth and sixthembodiments of the invention. In FIG. 7 a nanowhisker is shownupstanding from a substrate 70, having a gold catalytic particle 72 atits top, and being composed of a material 74, preferably a III-Vcompound such as GaAs, InAs, InP. The nanowhisker has its sides definedby (110) surfaces. The whisker is formed by the CBE method as describedabove. The nanowhisker is embedded in a surrounding layer 76 of a secondmaterial different from that of the first, but preferably also a III-Vcompound such as GaAs, InAs, InP. The material of region 74 may begallium arsenide, whereas material region 76 may be indium arsenide.Material region 76 is also grown by CBE, with conditions of'temperatureand/or pressure adjusted to support bulk growth, rather than VLS growth.

Preferably, nanowhiskers of III-V compounds are grown under group-IIIrich growth conditions (In, Ga, Al, B) that is for example an excess ofTEGa is used for CBE growth of whiskers containing Ga. This ensures thatthe outermost surface of the nanowhisker has a slight excess of thegroup III compound Ga, and is therefore intrinsically p-type. Theembedding layer 76 is InP, which embedding layer also grown under groupIII rich conditions to ensure a slight excess of In. The outermostsurfaces of the nanowhisker are (110) surfaces.

Thus, a pn-junction results by combining GaAs (p-type intrinsically)with InP (n-type intrinsically). Another example would be InAs, which isalmost degenerately n-type intrinsically.

By way of explanation, it is well understood that, at the free surfaceof a semiconductor, surface relaxation and surface reconstruction maytake place, to minimise free energy, in particular from chargeimbalance. Surface reconstruction may involve rearrangement of thecrystal lattice; this is particularly so for GaAs (111) surfaces.Further, surface trap states are created in the bulk band gap, and thisstrongly modifies the charge balance at the surface. This creates, inknown manner, a deformation of the band structure near the surface. Theband edges bend upwards so that the surface states cross the Fermi leveland start to empty, decreasing the surface charge density. The regionover which the bands are bent is termed the depletion region because ithas been depleted of mobile carriers. If the surface state density at asemiconductor surface has a high value, the band bending will saturate.At this point the Fermi level is said to be pinned by the surfacestates.

Since in this embodiment, the nanowhisker is grown under group III richconditions, the surface reconstruction creates, from these excess groupIII atoms, deep-level like defects, the energy position of which arerelated to the vacuum level, not to the band edges of the semiconductors(this corresponds to the situation for other deep level impurities inbulk III-V semiconductors).

Referring to FIG. 9, this shows the band gaps for a range of III-Vcompounds grown under group III rich conditions, with surface trapstates indicated by crosses occurring in the band gaps. It will be notedthat for all the compounds, the energy levels for the trap states areroughly equal, relative to vacuum level. This implies that pn junctionscan simply be created by Fermi Level pinning at an interface between twosuch materials.

Thus, the situation arises that the surface of a GaAs whisker is p-type,whereas the surface of an InP whisker is n-type. Further the surface oflayer 76 surrounding and embedding the whisker will have a conductivitygoverned by similar considerations. Thus Fermi Level pinning will ensurethat the surface of a surrounding InP layer is n-type; hence if thewhisker is GaAs, a pn junction is created by the Fermi Level Pinningeffects. The situation is shown in FIG. 10, where the relative levels ofthe band gaps of GaAs and InP are determined by Fermi Level Pinning,arising from the surface trap states.

In an alternative, where the whisker and surrounding layer are grown byMOVPE, then the MOVPE process has to be tuned to give Group III richconditions of growth.

In a further embodiment as shown in FIG. 8, a heterojunction 88 within ananowhisker 82 between an indium phosphide segment 84 and a galliumarsenide segment 86 assumes the character of a pn junction along a (001)or (100) crystal plane. This is because GaAs is intrinsically p-typewhereas indium phosphide is intrinsically n-type. The side facets of thewhisker are (111) planes that have many surface states which establish asurface Fermi level (pinned Fermi level) which is characteristic ofp-type or n-type semiconductor material, respectively. For nanowhiskersof a diameter of about 100 nm or less, there is insufficient diametraldistance to permit band bending in the interior of the whisker to alevel characteristic of the bulk semiconductor. Consequently, theconductivity type of each of the segments 82, 84 is determined by theFermi-level pinning produced by the surface states on the side facets ofeach segment. Accordingly, the heterojunction 88 becomes a pn junctionbetween the indium phosphide segment 84 and the gallium arsenide segment86 of the nanowhisker.

The skilled practitioner will, of course, recognize that theabove-described embodiments are illustrative of the present inventionand not limiting.

The invention claimed is:
 1. A semiconductor device, comprising: a III-Vsemiconductor nanowhisker of a first conductivity type standing uprightfrom a substrate; and a coaxial shell comprising a plurality of III-Vsemiconductor materials located around at least a portion of thenanowhisker; wherein: the coaxial shell has a thickness greater than 200nm; and an outer portion of the coaxial shell comprises a III-Vsemiconductor sheath of a second conductivity type.
 2. The device ofclaim 1, wherein the first conductivity type comprises n-type and thesecond conductivity type comprises p-type.
 3. The device of claim 1,wherein the first conductivity type comprises p-type and the secondconductivity type comprises n-type.
 4. The device of claim 1, whereinthe nanowhisker comprises a one dimensional nanoelement.
 5. The deviceof claim 1, wherein the substrate comprises a III-V semiconductorsubstrate.
 6. The device of claim 1, wherein the nanowhisker is formedon the substrate by a VLS process using a gold particle, and the goldparticle is located on top of the nanowhisker.